Objective: The purpose of this study was to evaluate the image quality in virtual monochromatic imaging (VMI) at 40 kilo-electron volts (keV) with three-dimensional iterative image reconstruction (3D-IIR). Methods: A phantom study and clinical study (31 patients) were performed with dual-energy CT (DECT). VMI at 40 keV was obtained and the images were reconstructed using filtered back projection (FBP), 50% adaptive statistical iterative reconstruction (ASiR), and 3D-IIR. We conducted subjective and objective evaluations of the image quality with each reconstruction technique. Results: The image contrast-to-noise ratio and image noise in both the clinical and phantom studies were significantly better with 3D-IIR than with 50% ASiR, and with 50% ASiR than with FBP (all, p < 0.05). The standard deviation and noise power spectra of the reconstructed images decreased in the order of 3D-IIR to 50% ASiR to FBP, while the modulation transfer function was maintained across the three reconstruction techniques. In most subjective evaluations in the clinical study, the image quality was significantly better with 3D-IIR than with 50% ASiR, and with 50% ASiR than with FBP (all, p < 0.001). Regarding the diagnostic acceptability, all images using 3D-IIR were evaluated as being fully or probably acceptable. Conclusions: The quality of VMI at 40 keV is improved by 3D-IIR, which allows the image noise to be reduced and structural details to be maintained. Advances in knowledge: The improvement of the image quality of VMI at 40 keV by 3D-IIR may increase the subjective acceptance in the clinical setting.
Since the introduction of 64-detector computed tomography (CT), coronary CT angiography (CCTA) has been used increasingly for the noninvasive detection of coronary artery disease with a high diagnostic accuracy 1, 2). However, arterial motion, which is one of the most common limitations in CCTA examination, renders many arterial segments "non-interpretable" 3-6). Coronary motion artifacts tend to be most pronounced at higher heart rates (HRs), and several studies have recommended an average HR no greater than 65 beats per minute (bpm) to obtain reliable image quality in CCTA and recommend the use of medications, if necessary, to control the HR 7-10). The Society of Cardiovascular Computed Tomography (SCCT) guidelines for the performance and acquisition of CCTA has reported that HR control has tangible benefits including not only image quality, but also the ability to use dose-reduction scan acquisitions. Dose-reduction scan acquisitions are not possible at higher HRs, and a target HR for a CCTA set at 60 bpm or less is usually appropriate 11). The multi-sector reconstruction algorithm (MSRA) retrospectively reconstructs images from different cardiac cycles (two to five), and this method can theoretically improve the effective temporal resolution. Meanwhile, the half-scan reconstruction algorithm (HSRA) reconstructs images from a single cardiac cycle. MSRA usually achieves a better temporal resolution than HSRA 12). On the other hand, a novel motion correction algorithm (MCA) has been recently introduced that compensates for coronary arterial motion using image sets reconstructed with HSRA at adjacent cardiac phases within a single cardiac cycle obtained using electrocardiographic (ECG)-gated CCTA scans 6). The use of MCA improves image quality, interpretability, and diagnostic accuracy in CCTA without the use of medications to control HR 6) .
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